clk: renesas: rcar-gen2: Switch Z clock to .determine_rate()

As the .round_rate() callback returns a long clock rate, it cannot
return clock rates that do not fit in signed long, but do fit in
unsigned long.  Hence switch the Z clock on R-Car Gen2 from the old
.round_rate() callback to the newer .determine_rate() callback, which
does not suffer from this limitation.

This includes implementing range checking.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20190830134515.11925-7-geert+renesas@glider.be